1. Field of the Invention
This invention relates to a viscous fluid coupling device used to control an engine cooling fan.
2. Description of the Related Art
A conventional viscous fluid coupling device of this type has been disclosed by Japanese Utility Model Unexamined Application No. Hei 3-114635 published in 1991. The viscous fluid coupling device comprises: a housing; a partition board dividing the internal space in the housing into a storage chamber and an operation chamber, the partition board having communicating holes to communicate the storage chamber with the operation chamber; a rotor which is provided in the operation chamber and is secured to one end portion of a drive shaft so as to rotate together with the drive shaft; a valve provided in the storage chamber to open and close the communicating holes; and a rod having one end secured to the valve and the other end secured to the inner peripheral portion of a spiral bimetal, the rod being rotatably supported by the housing, the housing and the rotor having labyrinth grooves in the surfaces which are confronted with each other. In the device, the communicating holes formed in the partition board are opened and closed by the valve provided in the storage chamber, to move the viscous fluid from the storage chamber into the operation chamber. When the temperature responding member senses a low temperature, the valve operates to close the communicating holes with the aid of the rod; and when it senses a high temperature, the valve operates to open the communicating holes.
The conventional viscous fluid coupling device thus constructed is disadvantageous in the following points:
When, for instance, in an automobile equipped with the device, the engine speed exceeds the regular range of speeds, then the temperatures sensed by the temperature responding member are all high, so that the valve operates to open the communicating holes at all times, and accordingly the viscous fluid is caused to flow from the storage chamber into the operation chamber. As a result, the engine speed is increased, and the fan speed is also increased. When the fan speed is increased to an excessively high value, the noise from the fan is increased, and the temperature of the internal viscous fluid is increased extremely by shearing. As a result, the viscous fluid is thermally deteriorated, and the device is therefore greatly lowered in performance.
When the engine speed has been increased beyond the regular range of speeds during high speed traveling, the speed of the fan is substantially maintained unchanged after being increased to a certain high value, whereas the temperature of the viscous fluid is kept increased. On the other hand, the engine can be cooled down with the wind produced during traveling. Therefore, it is substantially unnecessary to rotate the fan at high speed after the engine speed has exceeded the regular range of speeds.